Habitual sleep duration varies greatly in the general population. Inadequate sleep duration has negative consequences on health, including metabolic, neuroendocrine, and cognitive functions, as well as mood. Both, short and long sleep durations have been associated with increased morbidity and mortality. However, whether an individual's habitual sleep duration has a physiological basis is unknown. To answer this question, we propose to study 2 extreme phenotypic groups, short sleepers (habitual sleep duration <6 hours) and long sleepers (>9 hours). The duration and timing of sleep are regulated by an interaction between the circadian pacemaker, which programs daily cycles in sleep propensity, and the sleep homeostat, which tracks the increase of sleep pressure (i.e., sleep debt) during waking and its decrease during sleep. Either 1 of these regulatory systems might differ among individuals. Our previous studies indicate that short sleepers live under and tolerate higher sleep pressure, and they show a higher asymptote of the wake-dependent increase of theta/low frequency alpha activity (5-9 Hz) in the wake electroencephalogram (EEG), a marker of sleep pressure. Furthermore, the circadian pacemaker programs a longer biological night in long sleepers than short sleepers, as reflected in a longer nocturnal interval of the circadian rhythms in neuroendocrine function, body temperature and arousal. While such differences may perpetuate inter-individual disparities in sleep duration, it is not known whether they represent stable trait differences or are a consequence of the different photoperiod and/or sleep-wake histories. To differentiate between these possibilities, we designed a study in which short and long sleepers are exposed for 3 weeks to identical photoperiods and extended sleep opportunities before their spontaneous sleep duration, sleep structure, waking EEG, and waveform of the circadian rhythms in neuroendocrine function, body temperature and arousal are assessed. The goal of this project is to establish whether sleep duration[s] in short and long sleepers represent stable traits and are associated with differences in the duration of the biological night that is programmed by the circadian pacemaker and/or differences in the saturation level of a wake EEG marker of sleep pressure. The long-range goal is to understand whether sleep duration and sleep need are to be defined individually, and differ as a result of physiological and ultimately genetic disparities in sleep regulatory systems.